In many fields of automobile parts and airplane parts, etc., aluminum alloys are used to reduce weight. Aluminum alloys are superior in weight reduction, but thermal expansions thereof are large when the aluminum alloys are exposed to high temperatures.
For example, a technique is known in which an engine block for automobile engines is composed of an aluminum alloy to reduce weight. The engine block has a journal portion (journal bearing portion) for supporting a crankshaft.
As described above, in the case in which the engine block is composed of aluminum alloy to reduce weight, the journal portion is desirably composed of an aluminum alloy. This is because in the case in which there is a difference in thermal expansion rate among members of the engine block, engine performance is restricted due to undesirable stress generated in the engine block when the temperature of the engine is increased.
During the rotation action of the engine, heat generated in the cylinder is conducted to the overall engine block, and the temperature of the journal portion reaches 100 to 150 degrees C. In the case in which the journal portion is composed of an aluminum alloy, the aluminum alloy of the journal portion is expanded by the above temperature increase, so that the diameter of the bearing is slightly large.
The crank shaft is composed of an iron-type material to ensure the rigidity thereof, and naturally has a thermal expansion rate lower than that of aluminum alloy. Even if the journal portion reaches the above temperatures, the expansion thereof is small. Due to this, as the temperature of the engine increases, a gap is formed between the journal portion and the crank shaft, so that noise and vibration increase during high speed rotation thereof.
Techniques for inhibiting the expansion of the aluminum alloy are disclosed in Patent Documents 1 to 3. In the technique disclosed in Patent Document 1, an iron material having small thermal expansion is integrally cast in the portion of a member composed of aluminum, wherein the portion is required to inhibit thermal expansion. In the technique disclosed in Patent Document 2, an iron material is integrally cast with aluminum alloy after being subjected to plating processing and annealing processing. In the technique disclosed in Patent Document 3, a metal-based composite material composed of a reinforcing material and a matrix is obtained, the matrix on the surface of the metal-based composite material is melted by heating, and the composite material is integrally cast with melted light alloy containing 1 mass % or more of magnesium.
Patent Document 1 is Japanese Patent Unexamined (KOKAI) Publication No. S60-219436, Patent Document 2 is Japanese Patent Unexamined (KOKAI) Publication No. S58-112648, and Patent Document 3 is Japanese Patent Unexamined (KOKAI) Publication No. H5-337631.
However, in the techniques disclosed in Patent Documents 1 to 3, there are the following problems which need to be overcome. In the technique disclosed in Patent Document 1, the thermal expansion is inhibited but the adhesion characteristic between the aluminum alloy material and the iron material which is as the reinforcing material is not good, and the iron material peels from the aluminum alloy material. Due to this, the obtained material has low strength as a composite material, and the thermal expansion inhibition effect is decreased by the peeling. As a result, when the obtained material is used for a structure of a journal portion, noise and vibration are easily generated. Since an iron material is used, the obtained material is heavy, so that this is undesirable to reduce weight.
In the technique disclosed in Patent Document 2, although the adhesion characteristic between the aluminum alloy material and the iron material which is as the reinforcing material is increased, the production cost is high since plating processing and annealing processing are further required. In the technique disclosed in Patent Document 3, the metal based composite material is required to be separately produced, and heating processing therefore is required, so that the production cost is high.